Content optimization of a physical layer preamble

ABSTRACT

Embodiments of the present invention provide for content optimization of a physical layer preamble. In one embodiment of the invention, a method for encapsulating a payload for transmission through a network is disclosed. The method comprises the step of programming a legacy physical layer length value in a legacy physical layer preamble. The legacy physical layer preamble is configured such that it can be received by any legacy stations that may be on the network, and such that a separate physical layer length value can be derived from the legacy physical layer preamble. Using such a system, content optimization of a physical layer preamble is provided.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of priority from U.S. provisionalapplication 61/354,538, filed on Jun. 14, 2010, which is incorporatedherein in its entirety.

FIELD OF THE INVENTION

The invention relates generally to wireless local area networks (WLAN),and more specifically to WLAN protocols.

BACKGROUND OF THE INVENTION

When something is sent through the mail, it must be placed in a package,given the proper address, and moved through the system at the righttime. It is no different when a packet of information is sent through anetwork. The protocol for how a packet is sent through a network may bedefined by a network standard. The dominant standards for wireless localarea networks (WLANs) are those of the IEEE 802.11 family. The 802.11family of standards specify how information sent through the networkshould be packaged and how it should be addressed. A single 802.11specification may define the protocol for both a physical layer (PHY)and media access control layer (MAC) of the communication transmissionscheme. These two layers combine to prepare information for successfultransmission through a wireless network.

The PHY is the first layer in the seven-layer open systeminterconnection (OSI) model for layered communications. The PHY definesthe manner in which the raw zeros and ones that comprise a signal willbe grouped into code words or symbols and then converted into a physicalsignal. The PHY also defines how this physical signal is transmittedthrough a physical link such as a cable or the air. In a wirelessnetwork, some information regarding the transmitted signal, such asmodulation scheme and/or number of streams may be sent to one or morereceiving devices. In a wireless communication device, the stepsnecessary to implement the PHY layer packaging may be accomplished by aradio frequency processing radio component, and a base band processingcomponent. The radio modulates the signal in accordance with therelevant standard. Example modulation techniques include quadratureamplitude modulation (QAM), amplitude modulation (AM), and frequencymodulation (FM).

The MAC layer is a sub-layer of the second layer of the OSI model. TheMAC layer provides addressing and channel access control mechanisms sothat several stations can communicate within a network. Medium accessmust be controlled because if different stations within a networkbroadcast at-will, the air would be filled with conflicting signals.Just as a group of people sitting in a room need to learn to take turnstalking so that everyone can be heard, collision avoidance systems arenecessary so that multiple stations on a wireless network are nottalking at the same time thereby destroying the transmitted information.The channel access system used in the family of 802.11 standards iscalled carrier sense multiple access with collision avoidance mechanism(CSMA/CA). The operation of CSMA/CA can be described with reference toFIG. 1.

FIG. 1 illustrates four stations in a wireless network; station 100,station 101, station 102, and station 103. Whenever one of the stationswishes to access the medium it will sense the channel, basicallylistening to hear if someone else is talking. If the channel is busy,the station will wait a random amount of time, and then try to transmitagain. Range indicator 104 shows the area over which station 100 can beheard. Range indicator 105 likewise shows the area over which station101 can be heard. The configuration of stations and range indicators inFIG. 1 illustrates an initial problem with carrier sense collisionavoidance methods. Since station 103 is outside range indicator 104, ifstation 100 is transmitting a message to station 101, station 103 willbe unable to sense the transmission. Therefore, without an additionalsystem, station 103 will begin transmitting, and station 101 may beoverloaded and unable to understand the message from station 100.

The collision avoidance scheme used by the 802.11 family of standardsmay involve the transmission of two signals called request to send (RTS)and clear to send (CTS). The process can best be explained withreference again to FIG. 1. When station 100 wants to communicate withstation 101, it will first check the medium to make sure it is clear,and will then send out an RTS signal. The RTS signal will be received byany stations within range indicator 104, which includes both station 102and station 101. All stations on the network will have internal networkallocation vectors (NAVs) that they can set in response to informationcontained within the RTS signal. As long as a station's NAV is not zero,it will continue to wait and act as if the channel is taken. Inaddition, when station 101 receives an RTS signal that indicates it willbe the recipient of a packet, it will send out a CTS signal to allstations within range indicator 105, which includes station 103. Station103 will in turn set its NAV with information contained within the CTSsignal. In this way, collision avoidance is provided for thetransmission from station 100 to station 101. The information within theCTS and RTS signals that are used to set the stations' NAVs that isnecessary to orchestrate this system is delivered through headers thatare added on to the message packets as they travel down through the OSIlayers.

In the OSI model, each layer provides services to the layer above andreceives services from the layer below. Headers are added on to data asit moves down towards transmission through the physical medium such thatthe data is continually encapsulated and repackaged into a format thatthe recipient layer can operate on. With reference to FIG. 2, packet 200is comprised of PHY header 201, MAC header 202, miscellaneous headers203, and payload 204. PHY header 201 is added onto the signal as itmoves from the MAC layer into the PHY layer. Likewise, miscellaneousheaders 203 and MAC header 202 are added onto the signal as it movesinto the corresponding OSI layer. Headers are also commonly referred toas preambles.

A critical portion of PHY header 201 is the physical layer convergenceprotocol (PLOP). The PLOP contains, among other items, a bit stream thatrepresents the length of the packet, and the rate at which the packet isbeing transmitted in bits per second. The PHY uses this information toproperly detect the end of the packet, as this information will indicatethe time it will take for the signal to be sent through the physicalmedium. This is extremely important for WLAN that are deployed indoorswhere there are several different paths a signal can take as it bouncesaround inside a building. In such a multipath environment, knowing thelength of the signal is one way in which the PHY can screen out theeffects of these reflections.

MAC header 202 comprises, among other items, a frame control field whichtells the recipient station what kind of data to expect, a durationfield, and address fields. The address fields include a sender address,a recipient address, and an access point address. The address fields aidthe network in determining where the packet is going, and how to route apacket through the network. The duration field is the field that is usedby the MAC to set the NAV values for stations that receive the packetand header. Before sending a CTS or an RTS signal out, a station willdetermine how long the packet transaction will take, and will programthe duration field based on this calculation. It is through this processthat the MAC header information sets when an individual station canaccess the medium.

In order for CSMA/CA to work, stations from different iterations of the802.11 family need to be able to communicate to prior iterations. If alegacy station is unable to hear messages broadcasted by a modernstation, the whole system of carrier sensing will fail. With referenceto FIG. 1, assume that station 100 and station 101 are modern stationsand station 102 is a legacy station. In a system that did not accountfor backward compatibility, when station 100 began broadcasting forpurposes of sending a message to 101, station 102 would be unable tohear this broadcast. Therefore, when station 101 sends an acknowledgmentsignal back to station 100 to indicate that the information wasreceived, it is possible that station 102 may have begun broadcastingand fatally interfere with the exchange between station 100 and station101. If legacy stations broadcast independently of modern stations themedium will be taken up by chatter and the network will fail.

To allow for backwards compatibility, modern stations such as thoseconfigured to operate under the 802.11n and draft 802.11ac standards canbe configured to produce packets that contain a unique form of PHYheader. These PHY headers are comprised of two parts; a legacy physicallayer (PHY) preamble comprising a legacy rate field and a legacy PHYlength field, and a separate physical layer preamble that comprises thePLOP of the modern standard. The legacy PHY preamble is also sometimesreferred to as the legacy spoof preamble. The legacy PHY preamble isalways transmitted using six megabits per second (Mbps). At this rate,any 802.11 device using orthogonal-frequency division multiplexing(OFDM) will be able to decode the legacy PHY preamble. However, thelegacy stations will not be able to decode the separate physical layerpreamble, or any of the remaining portions of the packet. As a result,the legacy stations will keep quiet and be ultimately unaffected by themodern messages. This system will result in a quiet channel regardlessof modern packet transfer in a network comprising legacy systems. Thisis important because oftentimes a user or network administrator cannotcontrol the devices that are within the area of influence of a WLAN. Itis therefore not suitable to allow the introduction of a legacy systemto cause total failure of the network.

Payload 204 is the actual information that a user desires to transmit.Although PHY header 201 and its subsidiary legacy PHY preamble, MACheader 202, and miscellaneous headers 203 are absolutely necessary forpayload 204 to be transmitted through the network, they are wasted spacefrom the perspective of a perfectly efficient system. Although onecannot send a letter through the mail without an envelope, the weight ofthe envelope does affect the shipping cost of the letter. Given thatmodern wireless networks are able to send hundreds of thousands ofpackets per second, any minor decrease in the number of bits, orimprovement in the information content in a packet header could lead todramatic improvements in a network's overall efficiency and performance.

SUMMARY OF INVENTION

In one embodiment of the invention, a method for encapsulating a payloadfor transmission through a network is disclosed. In one step a legacyphysical layer length value in a legacy physical layer preamble isprogrammed. The legacy physical layer preamble is configured such that aseparate physical layer length value can be derived from it. The legacyphysical layer preamble is configured such that the legacy physicallayer preamble could be received by a legacy station on the network.Such an approach produces a method for encapsulating a payload fortransmission through a network with a content optimized physical layerpreamble.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagram of a prior art system that requires CSMA/CAfor communication within the system.

FIG. 2 illustrates a block diagram of a prior art data packet that canbe transmitted through a network.

FIG. 3 illustrates a flow chart of a method for transmitting a packetthat is in accordance with the present invention.

FIG. 4 illustrates a block diagram of an apparatus for transmitting asignal through a wireless network that is in accordance with the presentinvention.

FIG. 5 illustrates a flow chart of a method for receiving a packet thatis in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference now will be made in detail to embodiments of the disclosedinvention, one or more examples of which are illustrated in theaccompanying drawings. Each example is provided by way of explanation ofthe present technology, not as a limitation of the present technology.In fact, it will be apparent to those skilled in the art thatmodifications and variations can be made in the present technologywithout departing from the spirit and scope thereof. For instance,features illustrated or described as part of one embodiment may be usedwith another embodiment to yield a still further embodiment. Thus, it isintended that the present subject matter covers such modifications andvariations as come within the scope of the appended claims and theirequivalents.

The current approach to packet transmission in state of the art 802.11networks is inefficient. The physical layer preamble of packets on suchnetworks contains redundant information. Embodiments of the presentinvention work towards optimizing the information content of thephysical layer preamble of data packets, and the encapsulating headersof the packet as a whole. As an example, physical layer preambles for802.11n may contain superfluous information. An 802.11n physical layerpreamble contains a legacy preamble and a separate high-throughputphysical layer convergence protocol (HT-PLCP). A draft 802.11ac physicallayer preamble contains a legacy preamble and a separatevery-high-throughput physical layer convergence protocol (VHT-PLCP). Asmentioned previously, the legacy physical layer preambles are alwayscoded in 6 Mbps as this is the lowest transmission rate OFDM willsupport. All legacy devices will be able to receive and comprehend thelegacy preamble. The legacy preamble also contains a rate field and alength field in bytes. The HT-PLCP also contains a rate field and alength field where the length is specified in bytes. In either case, thelength field in the separate physical layer preamble is redundant giventhe length field present in the legacy physical layer preamble.

A specific embodiment of the invention wherein a payload is encapsulatedfor transmission through a network as a packet can be understood withreference to FIG. 3. In step 301 a legacy physical layer length value ina legacy physical layer preamble is programmed. The manner in which itis programmed is such that the information content of the physical layerpreamble of the packet is optimized. The legacy physical layer preambleis configured so that a separate physical layer length value can bederived from the legacy physical layer preamble. In addition, the legacyphysical layer preamble will still be able to serve the purpose of beingdetectable by any legacy stations connected to a network. Thereby, thelegacy physical layer preamble is able to serve two distinct purposeswithout increasing in size. As such, specific embodiments of theinvention will comprise smaller packet headers overall. As mentionedpreviously, header information is a necessary inefficiency in anynetwork architecture. Therefore, decreasing the size of these packetheaders can lead to significant gains in network efficiency.

In an exemplary embodiment, the packet would be sent through a networkoperating under the proposed 802.11ac standard. In this embodiment, step301 would comprise programming the legacy physical layer preamble of thepacket such that a recipient substation could derive the VHT-PLCP lengthvalue from it. In another exemplary embodiment, the packet would be sentthrough a network operating under the 802.11n standard. In thisembodiment, step 301 would comprise programming the legacy physicallayer preamble of the packet such that a recipient substation couldderive the HT-PLCP length value from it.

Since the separate physical layer preamble length value can be derivedfrom the legacy physical layer preamble, in a specific embodiment of theinvention, the field in the separate physical layer preamble thatcontains this value can be reappropriated for other uses. In step 302,this separate physical layer length field is reappropriated to carry analternative quantum of information. This alternative quantum ofinformation could either increase the information content andcommensurate functionality of the packet's header, or it could alleviatethe need for a quantum of information in a different section of thepacket's headers thereby decreasing the header size and increasing theefficiency of the network.

In a specific embodiment of the invention, the alternative quantum ofinformation is a media access control layer duration. This exemplaryembodiment of the invention would therefore allow increasedfunctionality of the media access control layer, or increase theefficiency of the media access control layer header by freeing up spacein that layer. In accordance with such an exemplary embodiment of theinvention, in step 303 a packet containing the payload information couldbe transmitted by a station such that a NAV of a recipient station couldbe set based on this quantum of information. This is in contrast toprior art methods where the NAV was set using information from the mediaaccess control layer.

In the exemplary embodiment where the packet would be sent through anetwork operating under the 802.11n standard, the procedure for derivingthe separate physical layer length value would involve a straightapplication of the value stored in the legacy physical layer preamblebecause they are both in units of bytes. All that would be necessary todetermine the payload length would be to delete the deterministic numberthat is the length of the separate physical layer preamble in bytes.Contrarily, in the exemplary embodiment where the packet would be sentthrough a network operating under draft 802.11ac, this same procedurewould be slightly more complex because the separate physical layerlength value in the draft 802.11ac VHT-PLCP is in units of symbols andnot in units of bytes.

In a specific embodiment where the physical layer length value isexpressed in units of bytes, and the separate physical layer length isin units of symbols, it is still possible to derive one from the other.In most standards, the legacy preamble has a set number of bytes persymbol, and the length of the separate physical layer preamble insymbols is again a deterministic number. Therefore, the separatephysical layer length value in symbols can be derived by dividing thelegacy physical layer length value by the legacy number of bytes persymbol and subtracting the number of symbols in the separate physicallayer preamble. When a packet is received, this operation can be used todetermine the separate physical layer length value based on the receivedlegacy physical layer length value. When a packet is transmitted thisoperation can be used in reverse to program the legacy physical layerlength value based on the size of the payload the transmitting stationwould like to transmit.

In a specific embodiment, the legacy number of bytes per symbol dividedby a legacy rate is always a factor of the size of the payload andseparate physical layer preamble added together. In an exemplaryembodiment, the payload is coded using normal guard interval coding, thelegacy number of bytes per symbol is 3 (i.e., 24 bits), and the legacyrate is 6 Mbps. In normal guard interval coding, each symbol takes up 4micro-seconds (μs). In this case, the legacy physical layer lengthdivided by the legacy rate will always be a multiple of 4 μs becausethere are 24 bits per symbol. Therefore, no matter how many symbols arein the payload and separate physical layer preamble, their total sizewill be a multiple of 4 μs, and no matter how many symbols are specifiedin the legacy physical layer preamble, the length specified will also bea multiple of 4 μs. This exemplary embodiment therefore has theadvantageous aspect of a one-to-one mapping of the legacy physical layerlength value to the size of the payload and separate physical layerpreamble combined.

In another specific embodiment, the legacy physical layer length valuedivided by the legacy rate is not always a factor of the length of thepayload and separate physical layer preamble added together. In anexemplary embodiment, the payload is coded using short guard intervalcoding, the legacy number of bytes per symbol is 3, and the legacy rateis 6 Mbps. In short guard internal coding, each symbol takes up 3.6 μs.This exemplary embodiment does not have a one-to-one mapping of thelegacy physical layer length value to the size of the payload andseparate physical layer preamble combined because the legacy physicallayer length values does not have sufficient resolution for thisencoding. For every 9 symbols indicated by the legacy physical layerlength values a single two-to-one mapping will be required.

An approach taken by a specific embodiment of the invention to solve theproblem introduced in the prior paragraph can be described withreference again to FIG. 3. In step 304, the separate physical layerpreamble and payload of a packet are coded using a short guard intervalcoding scheme. In step 305, the payload is padded with additional bitshaving zero values to eliminate the potential ambiguity. In anotherembodiment of the invention the potential ambiguity can be eliminated byusing an extra bit in the separate physical layer preamble that couldinstruct a recipient station as to which of the two potential lengthswas correct for the corresponding packet. In a still further embodimentof the invention, the potential ambiguity could be eliminated byapplying a modulo (%) 3 operation on the length value and selectingbetween the two potential lengths based on the result of that operation.

A specific embodiment of the invention comprising an apparatus forencapsulating a payload for transmission through a network can beunderstood with reference to FIG. 4. FIG. 4 is a block diagram of awireless communication device providing a client device with aconnection to a wireless network through client connection 401. Notethat the invention is not limited to use with wireless networks, andthat FIG. 4 displays a specific embodiment for purposes of describingthe invention. The wireless communication device comprises MAC controlsystem 402 which has a network allocation vector processing system 403.MAC control system 402 can transfer signals to and from clientconnection 401 and baseband processing system 404. The wirelesscommunication device additionally comprises radio 405 which can transfera signal to and from baseband processing system 404 and antenna 406.Baseband processing system 404 contains a legacy signal processor 407which is configured to derive a separate physical layer length valuefrom information contained in a legacy physical layer preamble.

In a specific embodiment of the invention, legacy signal processor 407is capable of converting a length value in a legacy physical layerpreamble in units of bytes into units of symbols. Upon receiving apacket, legacy signal processor 407 will read the legacy physical layerpreamble in units of bytes and will divide by the size of the symbol inthe payload in units of micro-seconds. Legacy signal processor 407 willthen subtract out a number of symbols in a separate physical layerpreamble. In this manner, the symbol count of the payload will bederived from the legacy physical layer preamble.

In another specific embodiment of the invention, a network allocationvector in network allocation vector processing unit 403 will be set by aquantum of information stored in a separate physical layer length field.Since legacy signal processor 407 can derive the information that isotherwise in a separate physical layer length field, this field can bereappropriated for purposes of carrying an alternative quantum ofinformation. This quantum of information can in turn be used by networkallocation vector processing unit 403 to set when the wirelesscommunication device can transmit. In FIG. 4, an exemplary embodiment isshown where switch 408 is controlled by network allocation vectorprocessing unit 403 which can set when the wireless communication devicecan send a signal out from client connection 401.

A specific embodiment of the invention wherein an encapsulated packed isreceived from a network can be understood with reference to FIG. 5. Instep 501, a packet with a legacy physical layer preamble is received bya station. The legacy physical layer preamble is configured so that anylegacy station connected to the network would be able to receive andcomprehend it. In step 502, a separate physical layer length value isderived from the legacy physical layer preamble.

In another specific embodiment of the invention, the separate physicallayer length field in the separate physical layer preamble isreappropriated for purposes of carrying an alternative quantum ofinformation. In accordance with such an embodiment, in step 503 the NAVof the station is set based on the alternative quantum of information.Therefore, if the recipient station is a legacy station it will complywith CSMA/CA based on information obtained from the legacy physicallayer preamble, and if the recipient station is a modern station it willbe able to properly obtain the separate physical layer length value fromthe legacy physical layer preamble. In effect, this specific embodimentcan accomplish the functionality of prior art systems with loweroverhead in terms of bit consumption, or it can provide for additionalfunctionality given the same level of overhead.

Although embodiments of the invention have been discussed primarily withrespect to specific embodiments thereof, other variations are possible.Various configurations of the described system may be used in place of,or in addition to, the configurations presented herein. For example,although the wireless communication device could be integrated into thesame chip as the client device or have any other spatial-architecturalrelationship with the client device. In addition, the wirelesscommunication device does not need to be a single system on a chip,because the individual components may be on separate substrates. Also,the radio could be an integrated radio. Although specific examples ofthe apparatus were given the apparatus disclosed comprises a set ofcomponents sufficient to execute any of the methods described herein.Also, the invention is not limited to use with an 802.11 standard as anynetworking standard benefits from decreased overhead and the otherbenefits of certain embodiments of this invention.

Those skilled in the art will appreciate that the foregoing descriptionis by way of example only, and is not intended to limit the invention.Nothing in the disclosure should indicate that the invention is limitedto systems that communicate through the use of charged particles, orsystems that rely mainly on silicon. Neither should anything in thedisclosure indicate that the invention is limited to wireless networks,or wireless networks where the transmission medium is air. Functions maybe performed by hardware or software, as desired. In general, anydiagrams presented are only intended to indicate one possibleconfiguration, and many variations are possible. Those skilled in theart will also appreciate that methods and systems consistent with thepresent invention are suitable for use in a wide range of applicationsencompassing any related to intersystem communication.

While the specification has been described in detail with respect tospecific and exemplary embodiments of the invention, it will beappreciated that those skilled in the art, upon attaining anunderstanding of the foregoing, may readily conceive of alterations to,variations of, and equivalents to these embodiments. These and othermodifications and variations to the present invention may be practicedby those skilled in the art, without departing from the spirit and scopeof the present invention, which is more particularly set forth in theappended claims.

What is claimed is:
 1. A method for encapsulating a payload fortransmission through a network comprising: programming a legacy physicallayer length value in a legacy physical layer preamble, wherein saidlegacy physical layer length value is expressed in units of bytes,wherein said legacy physical layer preamble is configured to be properlyreceived by a set of stations on a network, even if a legacy station isin said set of stations; reappropriating a separate physical layerlength field in a separate physical layer preamble to carry analternative quantum of information, wherein the separate physical layerlength field is originally appropriated to include a separate physicallayer length value that is derived by dividing the legacy physical layerlength value by a legacy number of bytes per symbol and subtracting anumber of symbols in the separate physical layer preamble; forming apacket for transmission on the network, wherein the packet includes thepayload, the legacy physical layer preamble and the separate physicallayer preamble; and transmitting the packet to the network.
 2. Themethod from claim 1, wherein said separate physical layer preamblecomprises a high-throughput physical layer convergence protocol or avery high-throughput physical layer convergence protocol.
 3. The methodfrom claim 1, wherein said alternative quantum of information comprisesa media access control layer duration.
 4. The method from claim 1,further comprising: receiving the transmitted packet with a recipientstation; and setting a network allocation vector in the recipientstation based on said alternative quantum of information.
 5. The methodfrom claim 1, wherein said network is compliant with one of IEEE 802.11nor draft IEEE 802.11ac standards.
 6. The method from claim 1, whereinsaid legacy physical layer length value in units of bits divided by alegacy rate in units of bits per second is always a factor of a size ofsaid payload.
 7. The method from claim 1, wherein said legacy physicallayer length value in units of bits divided by a legacy rate in units ofbits per second is not always a factor of a size of said payload.
 8. Themethod from claim 7, wherein: said legacy rate is six megabits persecond; said legacy physical layer length value in bytes is a multipleof three; and a symbol size in said packet is three point sixmicroseconds.
 9. The method from claim 7, further comprising the stepsof: coding said payload and said separate physical layer preamble usinga short guard interval coding scheme; padding said packet by a singlesymbol if adding an additional symbol to said packet would result in anambiguous packet size that would map to the same said legacy physicallayer length value as said packet without said additional symbol.
 10. Anapparatus for processing a packet including a legacy physical layerpreamble, a separate physical layer preamble and a payload, theapparatus comprising: a media access control system having a networkallocation vector processing system that is configured based on analternative quantum of information stored in a separate physical layerlength field of the separate physical layer preamble; and a basebandprocessing system coupled to the media access control system, whereinthe baseband processing system includes a legacy signal processor thatderives a separate physical layer length value from a legacy physicallayer length value of the legacy physical layer preamble, wherein saidlegacy physical layer length value is expressed in units of bytes, andsaid separate physical layer length value is derived by dividing saidlegacy physical layer length value by a legacy number of bytes persymbol and subtracting a number of symbols in the separate physicallayer preamble.
 11. The apparatus from claim 10, wherein said separatephysical layer length field is reappropriated from carrying the separatephysical layer length value to carrying the alternative quantum ofinformation.
 12. The apparatus from claim 10, wherein a networkallocation vector in said network allocation vector processing system isset by said alternative quantum of information.
 13. A method forreceiving a payload transferred through a network comprising: receivinga legacy physical layer preamble and a separate physical layer preambleof a packet containing said payload; and deriving a separate physicallayer length value from a legacy physical layer length value in saidlegacy physical layer preamble, wherein said legacy physical layerlength value is expressed in units of bytes, and said separate physicallayer length value is derived by dividing said legacy physical layerlength value by a legacy number of bytes per symbol and subtracting anumber of symbols in the separate physical layer preamble, wherein saidseparate physical layer preamble includes a separate physical layerlength field originally appropriated to carry the separate physicallayer length value, wherein said separate physical layer length field isreappropriated to carry an alternative quantum of information; whereinsaid legacy physical layer preamble is configured such that said legacyphysical layer preamble can be properly received by a set of allstations on a network, even if a legacy station is in said set of allstations.
 14. The method from claim 13, wherein deriving the separatephysical layer length value comprises converting the legacy physicallayer length value from units of bytes into units of symbols.
 15. Themethod from claim 14, further comprising setting a network allocationvector based on said alternative quantum of information carried in theseparate physical layer length field of the separate physical layerpreamble.